For the purpose of carrying out effective production making use of a robot, a plurality of industrial robots have been installed in a production system to simultaneously carry out plural kinds of work. In particular, in cooperative work for gripping, supporting, and assembling the same workpiece among a plurality of robots that simultaneously carry out plural kinds of work, a problem is an excessive load applied to the workpiece and the robots during the work. The excessive load is caused by shifts of the present gripping positions of the robots. Specifically, the excessive load occurs according to a position error from an ideal gripping position where no load is applied. The position error that causes the load includes a static position error due to an installation position error and a calibration error of the robots and a dynamic position error caused by a response delay of the robots. When force acting on the workpiece and the robots caused by a position posture relation among gripping positions of the respective plurality of robots exceeds a tolerance, it is possible to detect an abnormality, for example, by looking at a current value of a motor and safely stop the robots by performing the abnormality detection. For the purpose of reducing a mutual position error among robots in operation, a large number of technologies for improving synchronization accuracy of fundamental cyclic signals for each of operations have been proposed.
On the other hand, a technology for correcting position postures by force control when a shift of a position posture relation among gripping positions (finger positions) of fingers (end effectors) of a plurality of robots occurs has been developed. According to this technology, the gripping positions are corrected according to forces generated in the fingers and the fingers are controlled to move to positions where forces applied to the fingers are fit within a range of allowable forces. Such a correction technology for position postures can be roughly classified into a master slave system and an impedance control system. The master slave system is a system in which a certain robot is set as a master robot and the other robots are set as slave robots and the master robot carries out position control for an ideal track and the other slave robots operate following the master robot according to force control. As the master slave system, there is also a system called synchronous control system that does not include the force control as a configuration. This system is used in an industrial use as an inexpensive configuration not including a force sensor. A positional relation between the main robot and the slave robots is defined in advance and position command values for the slaves are calculated from the positional relation defined to correspond to a command value of the master (see, for example, Patent Literature 1). The impedance control system is a system for reversely calculating tracks of the robots from a desired track of a handling target object and, when the track is set as the command value, controlling the fingers to positions where an appropriate impedance characteristic is given to the forces applied to the fingers to apply desired motions to the respective robots (see, for example, Patent Literature 2).